Channel coherence time describes the average rate at which the wireless channel varies. Knowledge of the channel coherence time or the detection of a sudden change in the channel state may be used to improve the efficacy of several functions in a wireless system.
Beamforming techniques require updated information within the channel coherence time to improve transmission performance over a wireless link. Beamforming in a wireless channel requires knowledge of the channel at the transmitting device. The knowledge of the channel is to be applied to a beamformed signal within the channel coherence time; otherwise stale beamforming information may result in degraded performance on the wireless link. On the other hand, if beamforming information is obtained and updated at a rate much faster than the channel coherence time, valuable system processing resources are used while the same benefit to the wireless link could be achieved with fewer system processing resources.
The coherence time estimates are used to set the stale-out time of beamforming information and/or optimize the update rate of the beamforming weights to conserve system resources. A rate-selection algorithm is designed to converge to the optimal rate supported by the channel; the optimal bandwidth of the rate-selection loop or hysteresis is a function of the channel coherence time. In addition, wireless synchronization requires frame exchanges within the channel coherence interval. Channel coherence measurements may be used to filter measurement data obtained outside that interval or to set frame exchange parameters in synchronization measurements.
There are other application of channel coherence measurements that are useful to improve performance in a wireless communication system.